PARASITISM, ETC., ON THE SEA-SHORE 139 



The commonest forms belong to the genus Ancistrtiniy 

 and are characteristic dwellers among the gills, occurring 

 in many other Gasteropod molluscs also. The cilia are long 

 and powerful, the mouth opens posteriorly on one of the 

 narrow sides of the body. One of the surfaces is concave, 

 and is provided with a denser plume of cilia which work in 

 a determinate direction, exactly like those of Rotifers. These 

 cilia work in opposition to those of the gills, otherwise, of 

 course, the Infusorian would be swept away. Issel states 

 distinctly that Ancistrum is not parasitic, but probably 

 functions as a sweeper of the gills, ingesting excremental 

 products or loose cells. They might possibly become 

 harmful if they were too numerous. 



Parasitism. — ^The late Prof. Max Braun (1906) defined 

 parasites as " living organisms which, for the purpose of 

 obtaining food, take up their abode, temporarily or per- 

 manently, on or within other living organisms," but to us 

 this definition seems hardly adequate. We might extend 

 it somewhat as follows : " Parasites are organisms which 

 take up their abode, temporarily or permanently, upon or 

 within other organisms in order to feed upon them or to 

 share their food, and which are more or less specialised for 

 that purpose." The term parasitism, however, must 

 always be somewhat elastic in view of the varying forms 

 this habit takes. There are few animals that do not harbour 

 some unbidden guest, yet numerous as parasites are on land 

 they are still more numerous and varied under aquatic 

 (particularly marine) conditions. 



Parasites are without doubt descended from free- 

 living ancestors, and are generally considered to have been 

 evolved, by degrees, from forms that in the universal search 

 for food have been attracted to other forms by reason of 

 surplus food, body products, shelter, etc., and have thence 

 become more and more closely associated with such forms 

 until, finally, they have come to depend wholly upon the 

 food which they obtain at the expense of these hosts, fre- 

 quently to the victims' serious detriment. We have, then, 

 a long series of stages ranging from cases where parasitism 



